The present invention relates to an etalon used for an optical filter and so forth.
Optical filters for taking lights of a specific wavelength region out of lights having continuously or discontinuously distributed wavelength characteristics are used in various fields. Among such optical filters, filters utilizing etalons have been widely used from old days for refractometry of gases, precision measurement of length and interference spectroscopes. Moreover, etalons have been used basically as interference filters for forming a series of sharp transmission peaks in the fields of other measurement instruments, research of light scattering, performance analysis of lasers, development of laser lines of narrower band, astronomy and so forth (for example, Kudo and Uehara, Basic Optics, 3rd edition, pp.336-338, 1999, Gendai Kogakusha; and Oh, Optronics, No. 9, pp.167-171, 2001).
The etalon means two of multiple reflection planes of which gap is fixed with a spacer made of invar or the like used in, for example, a Fabry-Perot interferometer. In particular, an etalon of this type is called a Fabry-Perot""s etalon. Because it is difficult to maintain one reflection plane parallel to the other, the gap between the planes is fixed (Dictionary of Physics, Ed. by Editorial Committee of Dictionary of Physics, 1992, Baifukan).
There is also an etalon consisting one piece of transparent plate having two parallel surfaces so that the two surfaces of the transparent plate should serve as two of reflection planes. Such an etalon may be called a solid etalon. In such an etalon comprising one transparent plate, highly reflective coatings may be provided on the both surfaces of the transparent plate. The transparent plate of the etalon used in this case consists of optical glass, quartz glass, air or the like.
A transmission wavelength peak of light transmitting such an etalon as described above is observed when phases of light multiply reflected in the etalon become uniform at the outgoing plane of the etalon.
Meanwhile, demands for thinner etalons are increasing in various applications of etalons including measurement instruments. According to Kudo and Uehara, Basic Optics, 3rd edition, 1999, Gendai Kogakusha, transmission of the aforementioned solid etalon constituted with one transparent plate is given by the following equation.
T=1/(1+H sin2xcex4)xe2x80x83xe2x80x83(1)
In the formula:
H=4R/(1xe2x88x92R)2xe2x80x83xe2x80x83(2),
xcex4=(xcfx80/xcex)2nd cosixe2x88x92xcfx86xe2x80x83xe2x80x83(3),
T: Transmission of etalon,
R: Reflectance of etalon,
xcex: Wavelength,
n: Refractive index,
d: Thickness of transparent plate,
i: Angle of incidence,
xcfx86: Phase change due to reflection in etalon.
As for the characteristics of T, the maximum transmission is obtained at the time of sin xcex4=0, i.e., xcex4=mxcfx80 (m=1, 2, 3 . . . ). In the case of the angle of incidence i=0, the phase change xcfx86 should be xcfx86=0. In this case, xcex4 is defined by the following equation.
xcex4=(xcfx80/xcex)2ndxe2x80x83xe2x80x83(4)
Realization of a thinner etalon, which is one of the objects of the present invention, is equivalent to making the thickness d of the transparent plate in the equation (4) smaller. Since xcex4 is xcex4=mxcfx80 and xcex is determined by the desired wavelength, the way of making the thickness d of the transparent plate smaller, i.e., obtaining thinner etalon, is using a larger n.
Although a thinner etalon can be attempted by producing the etalon by using a material exhibiting a larger refractive index as described above, high thermal stability must be simultaneously attained. This can be expressed that, in other words, the value of nd in the equation (4) must not vary as far as possible, even though temperature variation occurs. To realize this is another object of the present invention. There is a tendency that conventional material exhibiting a high refractive index show poor thermal stability, and thus smaller thickness of etalon and higher thermal stability could not be satisfied simultaneously.
As described above, an object to be achieved by the present invention is to provide an etalon consisting of a material that simultaneously satisfies the following two of requirements.
(1) Larger Refractive index n
(2) Smaller variation of product of refractive index n and thickness d of transparent plate due to temperature variation.
The present invention was accomplished in order to solve such a problem, and an object of the present invention is to provide a solid etalon consisting of one transparent plate, which has a small thickness and exhibits high thermal stability.
The present invention accomplished in order to achieve the aforementioned objects provides an etalon comprising at least one transparent plate, wherein the transparent plate consists of lithium tantalate single crystal.
According to the present invention, the transparent plate constituting the etalon is made from lithium tantalate single crystal as described above. Lithium tantalate has a refractive index of 2.13 for an ordinary ray at a wavelength of 1550 nm, which satisfies a requirement that the refractive index should be 2 or more, and exhibits high thermal stability as represented by peak variation of 2.5 pm/xc2x0 C. for transmission wavelength due to temperature variation. Therefore, by preparing the transparent plate constituting the etalon from lithium tantalate single crystal, an etalon having a small thickness and exhibiting high thermal stability can be obtained.
The present invention also provides an etalon comprising at least one transparent plate, wherein the transparent plate consists of lithium niobate single crystal.
According to the present invention, the transparent plate constituting the etalon is also made from lithium niobate single crystal. Lithium niobate has a refractive index of 2.22 for an ordinary ray at a wavelength of 1550 nm, which satisfies the requirement that the refractive index should be 2 or more, and exhibits high thermal stability as represented by peak variation of 5 pm/xc2x0 C. for transmission wavelength due to temperature variation. Therefore, by preparing the transparent plate constituting the etalon from such a material, an etalon having a small thickness and exhibiting high thermal stability can be obtained.
In the aforementioned etalons, an angle between a normal of light incidence and outgoing plane of the transparent plate and a c-axis of the single crystal constituting the transparent plate according to the representation for hexagonal system is preferably 0xc2x0 to 10xc2x0.
As described above, both of lithium tantalate and lithium niobate satisfy the requirement of refractive index of 2 or more, and exhibit high thermal stability as represented by peak variation of less than 5 pm/xc2x0 C. for transmission due to temperature variation. When the direction of the c-axis of the single crystal and forward direction of light (referred to as xe2x80x9coptical axisxe2x80x9d hereinafter) are parallel to each other, in other words, when the normal of light incidence and outgoing plane of the etalon conforms to the optical axis and an angle between the optical axis and the c-axis of lithium tantalate or lithium niobate is 0xc2x0, good thermal stability can be obtained.
However, when an etalon is used, the median wavelength may be controlled by providing a certain angle between the optical axis and the light incidence and outgoing plane. Because lithium tantalate and lithium niobate show anisotropy, a larger angle between the optical axis and the normal of the light incidence and outgoing plane causes a larger peak variation of transmission wavelength due to temperature variation. This is caused by influences of the temperature dependency of refractive index, different coefficients of thermal expansion for different crystal orientations and abnormal light refractive index. However, if the angle is 0xc2x0 to 10xc2x0, desirably 0xc2x0 to 5xc2x0, it is possible to maintain high temperature stability even in such a case. Therefore, in the etalons of the present invention, high temperature stability can be maintained by controlling the angle between the normal of light incidence and outgoing plane of the transparent plate and the c-axis of the single crystal constituting the transparent plate according to the representation for hexagonal system to be 0xc2x0 to 10xc2x0.
In the etalons of the present invention, the transparent plate preferably consists of a single crystal grown by the Czochralski method.
If transparent plates that constitute etalons consist of a single crystal grown by the Czochralski method as described above, since single crystals of lithium tantalate and lithium niobate grown by the Czochralski method show good uniformity for refractive index and can be made as single crystals of a large diameter, the transparent plates are produced with good reproducibility and high yield, and thus etalons are produced at low cost.
The present invention also provides a method for producing an etalon, which comprises at least growing a single crystal of lithium tantalate or lithium niobate by the Czochralski method, producing a transparent plate from the single crystal and producing an etalon from the transparent plate.
If a single crystal of lithium tantalate or lithium niobate is grown by the Czochralski method as described above, a single crystal showing good uniformity for refractive index and having a large diameter can be obtained. Further, a single crystal of lithium tantalate or lithium niobate shows a high refractive index and high thermal stability. Therefore, if transparent plates are produced from such a single crystal and used for production of etalons, thin etalons showing high thermal stability can be produced with good reproducibility and high productivity.
In the aforementioned method, it is preferred that a single crystal having a diameter of 75 mm or more is grown, a single crystal plate having a diameter of 75 mm or more is produced from the single crystal, then the single crystal plate is divided into a plurality of transparent plates, and etalons are produced from the transparent plates.
In the processing of etalons, flatness and parallelism of surfaces affect the characteristics, and it is not rare that surface flatness of xcex/40 and parallelism of 1 second are required. The method of the present invention is a method for producing etalons of such high precision in a large scale, and it enables production of etalons of high precision in a large scale by dividing a large single crystal plate having a diameter of 75 mm or more and showing good uniformity for refractive index into a plurality of transparent plates and producing the etalons from the transparent plates. In this case, by producing the etalons from a portion of the single crystal plate of a large diameter showing high flatness and high parallelism, etalons having still better characteristics can be produced with good reproducibility.
As explained above, thin etalons having high thermal stability can be produced with high productivity according to the present invention. In addition, the etalons of the present invention can be used for various purposes such as optical filters thanks to their superior characteristics.